1. Field of the Invention
The present invention relates to a light-emitting element comprising an anode, a cathode, and a layer containing an organic compound (hereinafter, electroluminescent layer) that emits light upon being applied with an electric field, and more particularly such a light-emitting element that exhibits red emission. Also, the present inventions relate to a light-emitting device using the light-emitting element and an electric appliance using the light-emitting device.
2. Related Art
As a light-emitting element containing organic compounds as a light emitter, a light-emitting element that emits light by applying an electric field to pass an electric current across the light-emitting element is known. It is the emission mechanism that voltage is applied to an electroluminescent layer interposed between a pair of electrodes, and electrons injected from a cathode and holes injected from an anode are recombined each other within the electroluminescent layer to produce excited molecules (hereinafter, molecular exciton), then, the molecular exciton radiates energy as light while returning to the ground state. The excited states are known as a singlet excited state and a triplet excited state. It is considered that light emission can be obtained through either the excited states.
In such the light-emitting element, an electroluminescent layer is generally formed to have a thin film thickness of from approximately 100 to 200 nm. A light-emitting element does not require backlight, which is used for the conventional liquid crystal display device, since an electroluminescent layer emits light itself, that is, the light-emitting element is a self-luminous device. Therefore, it is highly advantageous that a light-emitting element can be manufactured into an extreme thin film and to be lightweight.
For example, in an electroluminescent layer having a thickness of approximately 100 nm, the time for the process of from the injection to the recombination of carriers takes approximately several ten nanoseconds in the light of the carrier mobility. Hence, the time required for the process of from the injection of carriers to the light emission of the electroluminescent layer is on the order of microsecond even if the process of the recombination of carriers is included therein. Thus, an extreme high response speed is also one of the advantages of a light-emitting element.
A light-emitting element containing organic compounds as a light emitter is a carrier injecting type device. Consequently, a light-emitting element is not required to be applied with a high alternating voltage as in the case with an inorganic electroluminescent element. A light-emitting element can be driven at a low direct-current voltage of from approximately several to ten several volts.
As noted above, a light-emitting element containing organic compounds as a light emitter has characteristics of thin and lightweight, high response speed, low direct-current voltage drive, and the like, and is attracted attention as a next generation's flat panel display device. Especially, a light-emitting device including such light-emitting elements arranged in a matrix configuration has superiority over the conventional liquid crystal display device in a wide viewing angle and high visibility.
In case that such light-emitting elements are utilized for a flat panel display or the like, emission color of light obtained from the light-emitting elements is required to be controlled to have desired color. As a method for controlling emission color of the light-emitting element, a method that a light-emitting layer is formed by a host material added with a small amount of guest material (also referred to as a dopant material) to obtain desired emission color derived from the guest material (hereinafter, doping system) is frequently used in recent years. (For example, see JP Patent 2,814,435.)
The doping system as typified by the foregoing JP Patent 2,814,435 can prevent concentration quenching of light-emitting molecules to obtain high luminance and high efficiency. Accordingly, the doping system is an effective method for emitting red emission material that is susceptible to be concentration quenched. For example, C. H. Chen et al. is disclosed that they synthesized red emission materials such as various 4-dicyanomethylene-4H-pyrane derivatives to use as a guest material in Macromolecular Symposia, No. 125, 49-58 (1997).
However, most light-emitting elements applied with such a doping system have a disadvantage that driving voltage is increased. Especially, it is known that a light-emitting element added with a red emission material as a guest material has a strong tendency to increase driving voltage. (For example, see Yoshiharu SATO, “The Japan Society of Applied Physics/Organic Molecular Electronics and Bioelectronics”, vol. 11, No. 1 (2000), 86-99).
Within a light-emitting element manufactured by a doping system, not only a guest material, but also a host material emits light, and so light emission from the device cannot be well controlled. As a result, color purity of the light emission may be deteriorated. The phenomenon is occurred when there is a large difference between excited energy of the host material and that of the guest material, and is common in a light-emitting element added with a red emission material as a guest material. It is considered that the phenomenon may be prevented by adding assist dopant having excited energy lying between the host material and the guest material. (For example, see Yuji HAMADA et al., Applied Physics Letters, Vol. 75, No. 12, 1682-1684 (1999).) It has been reported that the lifetime of a light-emitting element is improved by adding assist dopant and by doping materials that enable trapping holes. (For example, see T. K. Hatwar et al., Proceedings of the 10th International Workshop on Inorganic and Organic Electroluminescence (EL'00), 31-34.)
According to the method disclosed by Yuji HAMADA et al. and T. K. Hatwar et al, a driving voltage can not be reduced whereas luminescence with good color purity can be obtained.
A problem has arisen that a driving voltage is increased or color purity is deteriorated without controlling emission color in a light-emitting element manufactured by a doping system. It has been expected to solve the foregoing problems.